feat: Remove global db.Dbpool with dependency injection (Phase 0)

- Add Database struct in internal/db/database.go with Pool, Ctx, and RunMigrations()
- Update server.go to use Database struct with NewServerInstance()
- Add backend.go with InitBackend(), BackendRepo(), BackendCtx(), BackendPool()
- Update music.go and sync.go to use BackendRepo() and BackendCtx() instead of db.Dbpool/db.Ctx
- Update token_handler.go to accept pool parameter
- Update routes.go to use s.db.Pool for middleware
- Update cmd/main.go to use NewServerInstance() and HTTPServer()
- Update test_helpers.go to initialize backend with test database
- Update test files to use backend.BackendPool() and backend.BackendCtx()

Benefits:
- Easier to mock database for unit tests
- Follows Go best practices (dependency injection)
- Better architecture with explicit dependencies
- RunMigrations() replaces old Migrate_db() function

Note: Global db.Dbpool and db.Ctx still exist in dbHelper.go for backward compatibility
with test_helpers.go, but production code no longer uses them.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

internal/server/token_handler.go

@@ -7,10 +7,10 @@ import (
 	"strings"
 	"time"
 
-	"music-server/internal/db"
 	"music-server/internal/db/repository"
 	"music-server/internal/logging"
 
+	"github.com/jackc/pgx/v5/pgxpool"
 	"github.com/jackc/pgx/v5/pgtype"
 	"github.com/labstack/echo/v5"
 	"go.uber.org/zap"
@@ -30,13 +30,13 @@ type TokenResponse struct {
 
 // TokenHandler contains the database pool for token operations
 type TokenHandler struct {
-	pool *repository.Queries
+	pool *pgxpool.Pool
 }
 
 // NewTokenHandler creates a new token handler with database pool
-func NewTokenHandler() *TokenHandler {
+func NewTokenHandler(pool *pgxpool.Pool) *TokenHandler {
 	return &TokenHandler{
-		pool: repository.New(db.Dbpool),
+		pool: pool,
 	}
 }
 
@@ -84,7 +84,8 @@ func (h *TokenHandler) CreateTokenHandler(c *echo.Context) error {
 	clientType := req.ClientType
 
 	// Store in database using sqlc-generated repository
-	session, err := h.pool.CreateSession(c.Request().Context(), repository.CreateSessionParams{
+	queries := repository.New(h.pool)
+	session, err := queries.CreateSession(c.Request().Context(), repository.CreateSessionParams{
 		Token:      token,
 		IpAddress:  c.RealIP(),
 		UserAgent:  c.Request().UserAgent(),
@@ -132,7 +133,8 @@ func (h *TokenHandler) DeleteTokenHandler(c *echo.Context) error {
 	token := parts[1]
 
 	// Delete session using sqlc-generated repository
-	err := h.pool.DeleteSession(c.Request().Context(), token)
+	queries := repository.New(h.pool)
+	err := queries.DeleteSession(c.Request().Context(), token)
 	if err != nil {
 		logging.GetLogger().Error("Failed to delete session", zap.String("error", err.Error()))
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "Failed to invalidate token"})
@@ -158,7 +160,8 @@ func (h *TokenHandler) CleanupExpiredSessionsHandler(c *echo.Context) error {
 	// Verify token is valid first (using existing middleware)
 	// The middleware will have already validated the token
 
-	err := h.pool.DeleteExpiredSessions(c.Request().Context())
+	queries := repository.New(h.pool)
+	err := queries.DeleteExpiredSessions(c.Request().Context())
 	if err != nil {
 		logging.GetLogger().Error("Failed to cleanup sessions", zap.String("error", err.Error()))
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "Failed to cleanup sessions"})